They have vowed to hunt down and punish the Islamic State (IS) terrorists responsible for the Paris terrorist attacks that killed 129 people in an online "war".

Anonymous — the loose, unidentified collective of volunteer computer hackers or hacktivists — claimed it has already taken down 5,500 Twitter accounts linked to IS in the days since the attacks.

In a YouTube video that launched the Anonymous campaign #OpParis on Saturday, an Anonymous spokesperson wearing the familiar Guy Fawkes mask declared:

"To defend our values and our freedom, we are tracking down members of the terrorist group responsible for these attacks. We will not give up. We will not forgive. And we will do all that is necessary to end their actions."

But can Anonymous make any real difference in the fight against Islamic State? What can hacktivists achieve that air strikes in Syria or the sophisticated intelligence of Western governments cannot?

Quite a lot perhaps, some say.

The #OpParis campaign builds on the #OpISIS campaign launched early this year after the Charlie Hebdo attacks — also in Paris — when IS affiliated militants shot dead staff at the satirical newspaper.

Anonymous later claimed the Charlie Hebdo campaign exposed and destroyed more than 1,000 IS-linked online accounts, including Twitter accounts, email addresses and other websites.

How are Anonymous deleting IS Twitter accounts?

One analyst who has communicated extensively with Anonymous and many affiliated hacktivists in the months since, said he was able to verify the hacktivists' claims they took down thousands of IS-related Twitter accounts.

Various other internet tools, such as DOS attacks — or Denial of Service — have been used to infiltrate or disable IS-linked websites and email accounts.

IS has labelled Anonymous "idiots" via an online posted on a messaging service Telegram.

It is the thinking of these activists that if they can reduce ISIS' online visibility, that hinders its recruiting effort and reduces the power of its propaganda presence

Emerson Brooking, Think-Tank New America analyst

But it was Anonymous-affiliated hackers who succeeded in gaining vital intelligence that thwarted a terrorist attack in Tunisia, Mr Brooking said.

"I was shown an example of this by a US defence person, who served as an intermediary between the US intelligence community and one of these hacktivist groups," Mr Brooking said.

"The hacktivist group passed on to him intelligence pertaining to a possible attack in Tunisia, after the Soussa Beach massacre.

"He thought it warranted passing on to the FBI. He did so and the FBI passed it on to Tunisian authorities, and I believe it ended in the arrest of a dozen militants."

The new Anonymous campaign would adopt a similar approach. The person behind #OpParis — an anonymous Italian man — told the BBC that hacktivists plan to "finish off" IS altogether, not just on the internet.

"Our main goal in this operation is to identify the perpetrators of the Paris terror attacks and all terrorist organisations linked to them, acquire intel to dig deep into the roots of their manpower, disable their propaganda and stop their reach on social media, release their information to the public, and flag down any threat to mankind," the Italian man said.

Anonymous hacks disrupt IS effort to recruit more members online

But Mr Brooking said IS groups were usually only one step behind Anonymous hacktivists, using similar software and tactics.

"There is no way that you can stop someone from opening another account under another email. And indeed ISIS has developed all sorts of ways that regenerate accounts very quickly," Mr Brooking said.

"They have written their own computer programs to hide themselves from a lot of these Anonymous hunters.

"So the main thing this Anonymous group is doing is really wasting ISIS's time.

"It makes it harder and harder for legitimate ISIS fighters on the frontline — the people who have the greatest of propaganda draw — it makes it harder for them to actually sustain their web presence, because they are busy doing other things.

"They can't spend all day recreating the Twitter account that gets deleted four or five times a day."

Profile of a typical Anonymous hacker

Contrary to perceived stereotypes, Mr Brooking said most Anonymous hacktivists were well-adjusted members of society, with normal jobs — many with backgrounds in the military or security.

He said Anonymous and its affiliated groups could include several thousand people worldwide. But it may be a tiny core of just a couple of dozen hacktivists who carry out the #OpParis campaign.

"The typical Anonymous volunteer is not this hoodie-wearing internet savant in his mum's basement. Instead I think a lot of them were well adjusted functioning members of society, whose side habit just happened to be fighting the Islamic State on their nights and weekends," Mr Brooking said.

His cautions against suggestions that hacktivism could — even in the best case scenario — spell the end for Islamic State.

But Mr Brooking said he believed it had a valuable role to play; forcing IS and its supporters off the internet could seriously disrupt its ability to find new recruits.

"It is the thinking of these activists that if they can reduce ISIS's online visibility, that hinders its recruiting effort and reduces the power of its propaganda presence, both of which I find to be pretty compelling arguments," he said.

"But it is also important to note that this is something that they do in perpetuity. They readily acknowledge there is no end to this.

"If we can continue the effective kinetic assault we have now from the air; continue rallying these militia groups while simultaneously eliminating this ability of them to express themselves in this public forum and create these pieces of content that go viral and get at least heard about by tens of millions, I think we would go a very long way to getting rid of them for good."